1. Field of the Invention
The present invention relates to a projection type display using a light valve, particularly to a projection type display using a transmission type liquid crystal light valve.
2. Description of the Related Art
Among the projection type displays using the light valve for a light modulation, a projection type display using a liquid crystal light valve called a liquid crystal projector has the possibility to take the place of the CRT in the near future because the liquid crystal projector allows to display a fine and large image screen.
As the conventional projection type display used, there are a liquid crystal projector arranging a polarizing plate which has a transmission axis (or an absorption axis) oriented at 45 degrees diagonal with respect to the horizontal direction of a projection screen on the exiting side of a normally white type liquid crystal panel using a TN (twisted nematic) liquid crystal, and a liquid crystal projector having a transmission axis (or absorption axis) oriented parallel or perpendicular to the horizontal direction.
An exiting light from these conventional liquid crystal projectors is projected on the screen after being converted to a polarized light vibrating in the direction parallel to a major side or minor side of the display area of a rectangular shaped screen by a polarization converting device and so on. Also, one color of the three primary colors has a polarizing direction substantially orthogonal to the other two colors.
An example of a schematic structure of this conventional projection type display is briefly described with reference to FIG. 30. FIG. 30 shows the conventional projection type display using the transmission type liquid crystal light valve. A projection optical system of the projection type display is composed of a light source 1, liquid crystal light valves 21R, 21G and 21B, dichroic mirrors 4 and 6, a dichroic prism 14, a projection lens 16 and the like. The liquid crystal light valves 21R, 21G and 21B having a structure, which sandwiches both surfaces of liquid crystal panels 20R, 20G and 20B by polarizing plates, is used. The liquid crystal light valves 21R, 21G and 21B shown in FIG. 30, provide polarizing plates 20Rp, 20Gp and 20Bp on the exiting sides of the liquid crystal panels 20R, 20G and 20B respectively. On the incident side, a common polarization converting device 2 is arranged in the vicinity of the light source 1. Further, each of half-wave plates 20Rixe2x80x2, 20Gixe2x80x2 and 20Bixe2x80x2 is inserted on each incident side of the liquid crystal light valves 21R, 21G and 21B respectively and each of half-wave plates 20Ri, 20Gi and 20Bi is inserted on each of exiting sides respectively.
The three liquid crystal light valves 21R, 21G and 21B form images by modulating the intensity of the three primary colors of red (R), green (G) and blue (B) respectively according to image signals and transmit the images to the dichroic prism 14 which is, for example, a color synthesizing optical system. The liquid crystal light valve 21G is arranged at a position where the exiting light passes through the dichroic prism 14 and exits the prism 14. The liquid crystal light valve 21R is arranged at a position where the exiting light is reflected by a dichroic surface 14b of the dichroic prism 14 and exits the prism 14. Also, the liquid crystal light valve 21B is arranged at a position where the exiting light is reflected by a dichroic surface 14a of the dichroic prism 14 and exits the prism 14.
In the projection type display shown in FIG. 30, a white light illuminated from the light source 1 is incident on the dichroic mirror 4 as a linearly polarized light (p-polarization) having a polarizing direction (shown by arrows in the diagram) parallel to this page, after passing through the polarization converting device 2. The dichroic mirror 4 is structured so that a blue light is reflected thereby and other colors pass therethrough, where blue reflected by the dichroic mirror 4 is also reflected by a mirror 12 and is incident on the half-wave plate 20Bixe2x80x2. On the other hand, lights other than blue light pass through the dichroic mirror 4 and are incident on the next dichroic mirror 6. The polarizing direction of the blue light, which is incident on the half-wave plate 20Bixe2x80x2, is rotated 45 degrees and then the blue light is incident on the liquid crystal panel 20B which has substantially the same polarizing direction as the orientation direction of liquid crystal molecules on the incident side of the substrate side of the liquid crystal panel 20B in the blue liquid crystal light valve 21B.
On the other hand, the light passing through the dichroic mirror 4 is incident on the dichroic mirror 6 structured to reflect the green color and pass the red color. The green color reflected by the dichroic mirror 6 is incident on the half-wave plate 20Gixe2x80x2. The polarizing direction of the green color, which is incident on the half-wave plate 20Gixe2x80x2, is rotated 45 degrees by the half-wave plate 20Gixe2x80x2 and the green light is incident on the liquid crystal panel 20G, which has a polarizing direction substantially the same as the orientation direction of liquid crystal molecules on the substrate side of the incident side of the liquid crystal panel 20G in the liquid crystal light valve 21G for green. Also, the red light passing through the dichroic mirror 6 is incident on the half-wave plate 20Rixe2x80x2 after reflected by mirrors 8 and 10. The polarizing direction of the red light, which is incident on the half-wave plate 20Rixe2x80x2, is rotated 45 degrees by the half-wave plate 20Rixe2x80x2 and the red light is incident on the liquid crystal panel 20R which has a polarizing direction substantially the same as the orientation direction of the liquid crystal molecules on the substrate side of the incident side of the liquid crystal panel 20R in the liquid crystal light valve 21R for red.
A substrate on each exiting side of the liquid crystal panel 20R, 20G and 20B is rubbed in the direction orthogonal to the orientation direction of the liquid crystal molecules on the substrate side to which the light is incident. Therefore, a TN (twisted nematic) liquid crystal layer is formed on any of the liquid crystal panels 20R, 20G and 20B. Also, each of the liquid crystal panels 20R, 20G and 20B is an active matrix type liquid crystal panel, having a plurality of pixel areas where a p-Si TFT (a thin film transistor using poly-silicone for a channel layer) is formed as a switching device.
The blue light which is incident on the liquid crystal light valve 21B for blue is modulated by driving the switching device at the liquid crystal panel 20B and exits the polarizing plate 20Bp. The transmission axis of the polarizing plate 20Bp is set substantially in the same direction as the orientation direction of the liquid crystal molecules on the substrate side of the exiting side of the liquid crystal panel 20B. Therefore, the liquid crystal panel 20B is driven by a so-called normally white type, which obtains the greatest light transmission ratio under the condition where no voltage is applied to the TN liquid crystal layer of the pixel area. The blue light exiting the polarizing plate 20Bp is in turn incident on the half-wave plate 20Bi and the polarizing direction is converted to a polarizing direction perpendicular to this page, thereby being incident on the dichroic surface 14a as a s-polarized light and being reflected.
Similarly, the red light incident on the liquid crystal light valve 21R for red is also modulated by the driving of the switching device in the liquid crystal panel 20R and exits the polarizing plate 20Rp. The transmission axis of the polarizing plate 20Rp is also set substantially in the same direction as the orientation direction of the liquid crystal molecules on the substrate side of the light exiting side of the liquid crystal panel 20R. The liquid crystal panel 20R is driven by the so-called normally white type. The red light exiting the polarizing plate 20Rp is in turn incident on the half-wave plate 20Ri and the polarizing direction is converted to the polarizing direction perpendicular to this page, thereby being incident on the dichroic surface 14b as the s-polarized light and being reflected.
On the other hand, the green color incident on the liquid crystal light valve 21G for green is modulated by the driving of the switching device in the liquid crystal panel 20G and exits the polarizing plate 20Gp. The light transmission axis of the polarizing plate 20Gp is set substantially in the same direction as the orientation direction of the liquid crystal molecules on the substrate side of the exiting side of the liquid crystal panel 20G. Therefore, liquid crystal panel 20G is driven by the so-called normally white type. The green light exiting the polarizing plate 20Gp is in turn incident on the half-wave plate 20Gi and the polarizing direction is converted to a polarizing direction parallel to this page, thereby passing through the dichroic surfaces 14a and 14b as the p-polarized light.
In this way, the blue and red lights reflected by the dichroic prism 14 and the green light passing through inside the dichroic prism 14 are synthesized and exit to be enlarged by the projection lens 16 and project color images on the screen (the diagram is omitted).
In the conventional projection type display shown in FIG. 30, the light passing through the dichroic prism 14 is the p-polarized light and the light reflected by the prism 14 is the s-polarized light. In this manner, a deterioration of a separation specification and a spectrum synthetic characteristic at the dichroic surfaces 14a and 14b, which generate when the lights incident on the dichroic prism 14 are all s-polarized lights or each incident light is a combination of the s-polarized and the p-polarized lights, can be prevented. Thus, a cut-off specification of a reflection spectrum and a transmission spectrum at the dichroic surfaces 14a and 14b are improved, thereby resulting in an improvement in image quality. This is a technology described in Japanese Laid-open Patent Application No. 6-222321 and Japanese Laid-open Patent Application No.7-5410.
Meanwhile, along with a realization of a larger and finer projection area and a finer display of the recent projection type display, an accurate gradation display reducing an irregular color and a color shift of the display images on the screen has been particularly required. However, a situation is discovered by the present inventors that even if methods based on the above-mentioned prior art are used, the enough gradation display, the irregular color or the color shift can not be reduced. These disadvantages are produced because undesired light is illuminated to the p-Si TFT channel area provided at each pixel of the liquid crystal panel in the liquid crystal light valve, and a flow of a leak current is produced. As a result, an applied voltage to each pixel of the liquid panel varies and can not display the original gradation.
Here, a structure of the liquid crystal panel using the p-Si TFT as the switching device is described with reference to FIG. 31. FIG. 31 shows a partial lateral cross-sectional view of one pixel area of the liquid crystal panel. Without being limited to the liquid crystal panel of the projection type display, in the liquid crystal display, of an active matrix type generally using a switching device 104, the switching device 104 is formed for each pixel on an array substrate 100 formed by a transparent glass substrate, as shown in FIG. 31. A display electrode 110 made of a transparent electrode such as ITO (indium-tin-oxide) via an insulation film 108 is formed on the pixel area of the array substrate 100. Also, an opposing substrate 102 formed by the transparent glass substrate facing the array substrate 100 with a predetermined cell gap is provide and a common electrode 112 made of the transparent electrode such as the ITO or the like is formed on the array substrate side of the opposing substrate 102. A liquid crystal 106 is sealed in the TN liquid crystal layer between the array substrate 100 and the opposing substrate 102. Also, though the diagram is omitted, a orientation film made of, for example, polyimid or the like is formed at least on the contact surface between the transmission area of the array substrate 100 and the opposing substrate 102, and the TN liquid crystal layer, and the contact surface is rubbed to define the orientation direction of the above-mentioned liquid crystal molecules.
The switching device 104 shown in FIG. 31 is the p-Si TFT and n-type polysilicone layers 120 and 126 forming a drain area and a source area on the array substrate 100, and a polysilicone layer 124 functioning as a channel layer between the drain area and the source area are formed. On the polysilicone layer 124, a gate insulation film 122 made of, for example, SiO2 (Silicone Oxide film) is formed and a gate electrode 128 is formed on the gate insulation film 122. Further, on the n-type polysilicone layer 120 forming the source area, a source electrode 130 made of, for example, Al (Aluminum) is formed, and the display electrode 110 is formed on the n-type polysilicone layer 126 forming the drain area.
Also, on the opposing substrate 102 above the p-Si TFT, a shading film (black matrix) 114 to shield the incident light from outside the opposing substrate 102 is formed.
In the liquid crystal panel, such as the above structure, the array substrate 100 side is arranged facing the side of the dichroic prism 14 and the incident light from the light source 1 through the dichroic mirrors 4 and 6 is incident on the opposing substrate 102 side. In such an arranged structure of the liquid crystal panel, the incident light from the opposing substrate 102 side is shielded in order not to illuminate the switching device 104 in the liquid crystal panel by the shading film 114 provided on the opposing substrate 102 side.
However, when a stray light in the projection type display or an undesired light with a deviated wave-length is incident on the dichroic prism 14 and incident on the liquid crystal panel from the array substrate 100 side, the stray light or the undesired light illuminates the rear of the switching device 104 which does not form the shading film thereon. Therefore, a leak current is generated to set the switching device 104 to an on-state, and a voltage is applied between the display electrode 110 and the common electrode 112, therefore the orientation of the liquid crystal molecules at the area varies and an accurate gradation display can not be performed. Particularly, with the p-Si TFT superior in response, the leak current due to a shorter wavelength light can not be neglected. If the light separation characteristic of the dichroic prism 14 is perfect, the undesired shorter wavelength light incident on the dichroic prism 14 from the liquid crystal light valve 21B for blue, from which the light in blue band on the shorter wavelength side exits, is almost reflected by the dichroic surface 14a and is not incident on the liquid crystal light valves 21R and 21G for red and green, so that no undesired leak current is produced to each TFT.
However, when the undesired shorter wavelength from the liquid crystal light valves 21R and 21G for red and green, from which the light in the red band or the green band longer in wavelength than the blue color band exits, is incident on the dichroic prism 14, the undesired shorter wavelength passes through the dichroic surface 14b or is reflected thereby, and is incident on the liquid crystal light valve 21B for blue. As a result, the undesired leak current is generated to the p-Si TFT of the liquid crystal light valve 21B for blue.
FIG. 32 shows a light resistance of the p-Si TFT. A lateral axis indicates the quantity of white light incident on the liquid crystal panel in logarithmic display and a vertical axis indicates the extent of an error of the gradation display of the liquid crystal panel based on the leak current generated at the p-Si TFT as the quantity of leak. The quantity of the incident white light is the total quantity of red, green, and blue and the ratio of the quantity of light is (red:green:blue=3:12:1). In the diagram, a leak quantity characteristic at the liquid crystal light valve 21B for blue is indicated by a thick solid line (B), a leak quantity characteristic at the liquid crystal light valve 21R for red is indicated by a dashed line, and a leak quantity characteristic at the liquid crystal light valve 21G for green is indicated by a thin solid line. As is clear from FIG. 32, it will be understood that as with any liquid crystal light valve, although the quantity of leak increases along with an increase of the quantity of light incident on the liquid crystal panel, the increase of the quantity of leak at the liquid crystal light valve 21B for blue is particularly remarkable. For example, taking an example of a projection type display having the quantity of light incident on the liquid crystal panel equal to 50000001xc3x97, the quantity of leak at the liquid crystal light valve 21B for blue is 1.25 which is larger than the quantity of light of the liquid crystal light valve 21R or 21G for red or green which is 0.7xcx9c0.75, thereby producing a larger gradation change. Thus, when a balance between the gradations of red, green, and blue is broken without any relationship with the original modulation signal by the influence of the undesired light, a problem of a deteriorating display quality occurs because a light color synthesized by the dichroic prism 14 differs from a desired color.
In addition, the light emitted from the conventional liquid crystal projector to the screen is converted by the polarization converting device and the like. One of the three primary colors, for example, is emitted as a polarized light vibrating in the horizontal direction with respect to the screen while the other two colors are emitted as polarized lights vibrating in the vertical direction with respect to the screen. However, since almost all screens, including a projection screen composed of a combination of a lenticular lens and a Fresnel lens, differ in scattering characteristic depending upon the polarizing direction, the white balance is broken, thereby causing problems such as a generation of the irregular color on the screen or the color shift which changes the color depending upon the viewing position.
Also, in the projection type display shown in FIG. 30, since images are synthesized by the dichroic prism 14 which is a block of glass, none of the transmission, distortion of the reflection surface, and arrangement displacement is produced as compared with the dicroric mirror composed of plates and therefore the generation of a picture displacement can be prevented. However, another problem, that of the light path lengths of colors from each of the light source 1 to each of the light valves 21R, 21G, and 21B for respective colors become different, is produced. In FIG. 30, since the light path of red is longer than the light paths of green and blue, the balance of the quantity of red, green, and blue lights is deviated, so producing a problem that chromaticity is deviated when the combination display such as a white display by the projector is performed.
On the other hand, as shown in FIG. 33, after separating the light from the light source 1 into red, green, and blue by two dichroic mirrors 140 and 142 and a total reflection mirror 144, the conventional projection type display modulates the image by illuminating the three lights on the three liquid crystal panels 156R, 156G and 156B respectively. Then, after the three colors are synthesized into an image by two dichroic mirrors 148 and 150 and a total reflection mirror 146, the three colors are enlarged and projected by the projection lens. It should be noted that the respective light valves 21R, 21G and 21B of the projection type display shown in FIG. 33 are composed of condenser lenses 152R, 152G and 152B, incident polarizing plates 154R, 154G and 154B, liquid crystal panels 156R, 156G and 156B, and exiting polarizing plates 158R, 158G and 158B.
A projection type display shown in FIG. 33 is problematic in that the image displacement at the time of a image synthesis and a defect called a pixel displacement on the enlarged projection image created by the projection lens are easily produced, when distortions on the transmission or reflection surface, or a displacement produced during the arrangement, generate to the dichroic mirror 148 and 150 or to the total reflection mirror 146, which synthesize the images for respective colors modulated in image by the respective light valves 21R, 21G, and 21B.
To prevent these mirror distortions and displacements, a thicker mirror or an improvement of the mirror mounting method is required. However, when the mirror is made thicker, the problem, that an aberration of the transmission image light is enlarged, is produced. Also, the improvement of the mirror mounting method produces other problems such as high accuracy and high price of fixtures.
In addition, the projection type display for displaying the display of a personal computer, video or the like on the large screen is required to have finer display pixels along with finer signal source. Therefore, as has been described, a method which uses a white light power source in the projection type display, separates the white light into the three primary colors and synthesizes is the mainstream. The liquid crystal panel is required to be formed as small as possible for finer display and an improvement in portability. A greater pixel number is required as well. In proportion to the miniaturization of the pixel pitch, an improvement in display quality is required by making an astigmatism as small as possible. Therefore, a projection type display which sandwiches a dichroic mirror for total color synthesis by glasses to form a prism is also proposed (Japanese Patent Application No. 10-120568). FIG. 34 shows this proposed projection type display. A white light of a non-polarized light emitted from a light source 221 is separated by dichroic mirrors 224 and 229, and each of the separated green, red and blue lights is incident on liquid crystal panel 231, 230 or 226 by passing through each polarizing plate. Each of the red, green, and blue lights becomes a p-polarized light passing through the exit side polarizing plate after being space-modulated by each liquid crystal panel. Then, each light is synthesized through a dichroic mirror 228, a mirror 233 and a dichroic prism 234, and then reaches a projection lens 235.
In a p-polarized light synthesis, a reflection characteristic of the dichroic prism is low due to the character of the dichroic prism 234, thereby causing problems that the quantity of the red and blue lights reflected by the dichroic prism 234 reduces and the display quality of the projected images drops due to a broken balance of the quantity of each color light.
An object of the present invention is to provide a projection type display which is superior in gradation display and high in quality.
The above object is achieved by a projection type display comprising three light valves which have a polarizing plate at least on each exiting side thereof to modulate and emit each light of color components of red, green, and blue, a color synthesizing optical system to synthesize respective exiting lights from the light valves, and a polarization converting unit which sets a polarizing direction of the exiting light of the blue component among the three respective exiting lights substantially orthogonal to polarizing directions of other two exiting lights.
According to this structure, even if the exiting light of the red or green component is incident on the color synthesizing optical system having a linear polarization with a predetermined polarizing direction and the exiting light includes the undesired light (unnecessary light) having a shorter wavelength, only the linearly polarized light having a polarizing direction substantially orthogonal to the predetermined polarizing direction can pass through the light valve for blue to emit the light of the blue component due to a polarizing plate 20Bp, thereby preventing an entry of the undesired light. Therefore, when the light valve has a switching device for a light modulation, the light valve prevents a leak current from being generated and a superior gradation display can be performed.
Also, When an exiting light of the blue component including the undesired light is incident on the color synthesizing optical system, the undesired light exits the color synthesizing optical system together with the blue component because the blue component is on the relatively shorter wavelength side. Originally, a deterioration of a display quality by the undesired light is low. However, only the linearly polarized light having the polarizing direction substantially orthogonal to the predetermined polarizing direction can pass the light valves for red and green to emit the red or green component due to a polarizing plate 20Rp or 20Gp, therefore the entry of the undesired light can be surely prevented.
One of the aspects in the projection type display of this invention, the light valve comprises an active matrix type liquid crystal panel having a plurality of switching devices formed at respective pixel areas. Also, the switching device is made of polysilicone TFT. Furthermore, the liquid crystal panel is a transmission type liquid crystal panel to modulate the transmission light. According to this structure, a high quality image display can be performed to the switching device in the liquid crystal panel to modulate the incident light for obtaining desired images by preventing the leak current from being generated by the undesired light. Since the polysilicone TFT has low light resistance against the shorter wavelength light, the light leak of the liquid crystal panel using the polysilicone for the switching device is suppressed to minimum and preferable images with a quick response can be achieved. Also, since this invention operates extremely effective for preventing the undesired light which is incident from the rear side of the switching device provided in the transmission type liquid crystal panel, a shielding film and the like to intercept the undesired light are not required to be formed on the rear side of the transmission type liquid crystal panel, so that the conventional transmission type liquid crystal panel can be used as a structural element of the light valve.
Further, a projection type display of the present invention has a light source emitting a light including respective components of red, green, and blue, and a dichroic mirror to separate the light from the source into the respective color components. Among the respective polarizing plates, the polarizing plate to pass the incident light of the blue component is arranged to have the transmission axis substantially orthogonal to a transmission axis of the polarizing plate to pass the other two incident lights. A polarization converting unit has a feature that the emitted light from the light source is incident on the respective light valves after setting the polarizing direction of the blue component of light orthogonal to the polarizing directions of the other two color component lights. According to this structure, an advantage of the present invention that any optical devices are not required to be arranged between the polarizing plate provided on the exiting side of each light valve and the color synthesizing optical system can be achieved.
On the other hand, by arranging the transmission axes of the respective polarizing directions parallel to each other viewed from the traveling direction of each transmission axis, the polarization converting unit can also have a polarization converting device provided between the polarizing plate to pass the exiting light of the blue component and the color synthesizing optical system. On the other hand, arranging the transmission axes of the respective polarizing directions parallel to each other viewed from the traveling direction of each transmission axis, the polarization converting unit can have a polarization converting device provide between the color synthesizing optical system and each of two polarizing plates to pass the exiting lights of the red and green components. These structures have an advantage that the effect of the present invention can be achieved only by arranging the polarization converting device between a polarizing plate 1 or 2 provided on the exiting side of each valve and the color synthesizing optical system.
Also, the projection type display of the present invention is arranged to set transmission axes of the respective polarizing plate parallel to each other viewed from the traveling direction of each transmission light, where the polarization converting unit has the polarization converting device provided between the color synthesizing optical system and each of the polarizing plates to set the polarizing direction of the exiting light of the blue component orthogonal to the polarizing direction of the exiting light of the red and green components.
In the above projection type display, the polarization converting device is a half-wave plate. Also, the polarization converting device can be a liquid crystal panel which sets the polarizing direction of the exiting light of the blue component orthogonal to the polarizing directions of the exiting lights of the other two components. Further, in the projection type display of the present invention, the polarization converting unit can have the liquid crystal panel provided between the color synthesizing optical system and each of the polarizing plates to pass the exiting light of each color component.
Also, in the projection type display of the present invention, the light valve emitting the modulated light of the blue component can be arranged at the position where the exiting light passes through the color synthesizing optical system. In this case, a better-balanced color synthesis can be realized when the exiting light of blue has the p-polarization with respect to the color synthesizing optical system and the other two lights have the s-polarization, thereby resulting in a high quality image display.
Above object is achieved by a projection type display which comprises three light valves having the polarizing plate at least on each exiting side, modulating and emitting light of each color component of red, green, and blue respectively, the color synthesizing optical system to synthesize each exiting light from each light valve, a projection lens to project the synthesized light on a screen, and a polarization converting unit to convert the light polarization of each color component for equalizing a ratio of the quantity of light at least between the quantity of light in the parallel direction and the vertical direction for each color on the screen.
In the projection type display of the present invention, the polarization converting unit converts each synthesized light to a circularly polarized light. The polarization converting unit has a quarter-wave plate arranged on the exiting side of the color synthesizing optical system. Also, the quarter-wave plate has an optic axis of substantially 45 degrees with respect to an transmission axis or absorption axis of the polarizing plate.
According to this structure, though each of image lights synthesized by the color synthesizing optical system is linearly polarized light, each of the image light is converted to the circularly polarized light by the polarization converting unit, before the image light is incident on a projection lens. Therefore, the image light which is scattered by the projection lens and then exits are projected on a screen as the circularly polarized light. For example, when the screen is composed of the combination of a Flesnel lens and a lenticular lens, a problem that a color tone of the images varies depending on viewing angles due to the difference of refracting characteristics at the screen if the respective lights of red, green, and blue are linearly polarized lights, is occurred. However, the image light transmitted from this structure of the present invention does not occur any such problems, therefore providing high quality images. Also, the quarter-wave plate can be used as the linear polarization converting devise.
One of the aspects in the projection type display of the present invention, the polarization converting unit converts each of the synthesized lights to a linearly polarized light so that a bisector of an angle, formed by the polarizing directions of the lights between one color component and the other color components, is substantially identical to one of the horizontal line and the vertical line. Also, the polarizing direction of the light of one color component is orthogonal to the polarizing directions of the lights of the other color components by the polarization converting unit. Further, the polarization converting unit has a half-wave plate arranged on the exiting side of the color synthesizing optical system. Furthermore, the half-wave plate has an optic axis which is inclined by approximately 22.5 degrees with respect to the transmission axis or the absorption axis of the polarizing plate.
Also, a projection type display of the present invention has the screen on which the light exiting the projection lens is incident, wherein the screen has the Flesnel lens and the lenticular lens. From this structure, the ratio of the intensity of the light between the horizontal component and the vertical component for the three primary colors can be set substantially identical in the three primary colors. So, a screen light distribution characteristic of the three primary colors become identical on the screen, thereby displaying the high quality images with a high illumination without color variations and color shifts.
Next, the principle of a projection type display without a pixel displacement and the color variations realized according to the present invention is described with reference to FIG. 13. This projection type display includes the light source 1, a first dichroic mirror 140xe2x80x2, a second dichroic mirror 142xe2x80x2, a total reflection mirror 144xe2x80x2, respective light valves 21R, 21G and 21B, a first dichroic prism 160, a second dichroic prism 162, a total reflection prism 164 and the projection lens 16.
In this invention, light paths in the color separating optical system from the light source 1 to the respective light valves 21R, 21G and 21B for each color can be made substantially identical with the conventional paths shown in FIG. 33, therefore the chromaticity variations caused by the light path difference in the projection type display shown in FIG. 30 of the conventional example are not produced.
The color synthesizing optical system from the respective light valves 21R, 21G and 21B to the projection lens 16 is composed of only the first dichroic prism 160, the second dichroic prism 162, and the total reflection prism 164, so that the distortion on the mirror surface and the position displacement is hard to occur, thereby preventing the pixel displacement produced in the conventional example in FIG. 33.
However, the prism using a glass block has a problem that the price is more expensive than the dichroic mirror having a filter on a glass plate. Particularly, when a band-pass filter which passes or reflects only the light in the wavelength range having a visible light area is used, a film structure to obtain preferable filter characteristics becomes complicated and more expensive. Further, a problem that a manufacturing yield rate decreases to satisfy the specifications is produced, thereby resulting in a higher price. Therefore, being different from that in FIG. 33, in this invention, green and red are first synthesized by the first dichroic prism 160 and then synthesized with blue by the second dichroic prism 162. That is, the color separation and synthesis is performed only by the low-pass filter or the high-pass filter. The above object is achieved by a projection type display comprising a color separating optical system which has at least a dichroic mirror and separates an emitted light from a light source into respective lights of color components of red, green and blue, and three light valves which have at least a polarizing plate on each light exiting side thereof, modulate the light of each color component of red, green, and blue, and a color synthesizing optical system to synthesize each exiting light from each light valve, wherein the color synthesizing optical system comprises at least one dichroic mirror contacting both surfaces with solid or liquid, wherein the dichroic mirror of the color separating optical system and the color synthesizing optical system comprises a low-pass filter or a high-pass filter. By this structure, a projection optical system, which is small in cost increase, is realized without the above chromaticity variations and pixel displacement.
One of the aspects in the projection type display of the present invention, a light reflected by the dichroic prism within the synthesized light is made to be the s-polarized light to remove the reflection of the p-polarized light having a low dichroic prism reflection characteristic. When the light passing through the dichroic prism is converted to the s-polarized light as is the case of the reflection light, a cut-off wavelength to switch the transmission and reflection on the dichroic surface is deviated due to light having an angle with respect to the optical axis or the manufacturing error of the dichroic prism. To prevent these deviations of the projection display colors caused by the deviation of the reflection and transmission color in the dichroic prism, the transmission light is made insensitive to the deviation of the cut-off wavelength of the dichroic prism, by converting the polarizing direction of the transmission light having a wavelength close to the wavelength of light reflected by the dichroic prism to the p-polarization.
Thus, by converting the reflection light in the dichroic prism to the s-polarized light and the transmission light close to this reflection light to the p-polarized light, a color synthesis which is not dependent on the deviations of the dichroic prism cut-off wavelength is achieved, thereby improving the display quality of the images.
Also, along with above conversions, the brighter projection display images are obtained by the adjustment of the polarizing directions during the color separation and synthesis by designing the optical members matching to the above conversion.